Contrastación de dimensiones

7.1 GENERAL

Foundation works should be carried out such that :

(a) the foundation is constructed in accordance with the plans approved by the Building Authority;

(b) suitable methods and sequence are adopted so as not to render inadequate the margin of safety of, or impair the stability of, or cause damage to any building, structure, land, street or services; and

(c) adequate precautionary and protective measures are provided to assure the safety of the workers on site, all persons near the site, and adjacent buildings, structures, lands, streets and services.

7.1.1 GENERAL REQUIREMENTS

All foundation works, except where the penetration depth of the foundation element does not exceed 3 m, are specialized work in the foundation category and shall be carried out by a Registered Specialist Contractor (Foundation Works).

Foundation works should comply with :

(a) the requirements of site safety supervision as stipulated in the Code of Practice for Site Safety Supervision and Technical Memorandum for Supervision Plans; and

(b) the requirements of quality supervision (see Clause 7.1.2).

7.1.2 QUALITY SUPERVISION FOR FOUNDATION WORKS

The foundations are important structural elements of a building or structure. Adequate supervision should be provided to ensure that the quality of the foundation works in respect of materials, general arrangement, installation procedures, workmanship and testing are up to the required standards and conform with all relevant provisions of this Code of Practice. Reference should be made to the relevant practice notes for requirements of quality supervision for foundation works.

7.1.3 CONSTRUCTION MATERIALS (1) Concrete and grout

Sampling of concrete and compression testing of concrete test cubes should be carried out in accordance with the methods specified in CS1:1990. Testing should be carried out by a HOKLAS accredited laboratory. The test results should be reported on a HOKLAS Endorsed Test Certificate. Concreting for cast-in-situ piles should be in one continuous operation.

(2) Reinforcement

Sampling and testing of steel reinforcement should be carried out in accordance with the methods specified in CS2:1990. Testing should be carried out by a HOKLAS accredited laboratory. The test results should be reported on a HOKLAS Endorsed Test Certificate.

(3) Steel piles

A copy of the mill certificate of the structural steel used should be submitted to Buildings Department together with a statement signed by the RSE to confirm that the requirement of chemical composition and mechanical properties appropriate to the type of steel have been complied with.

7.1.4 EXCAVATION

Foundation works often require excavation, which, even if shallow, can be dangerous if not properly designed. Adequate precautionary and protective measures should be provided to ensure that the excavation works are properly protected and the foundation works are carried out safely.

7.2 EFFECT OF FOUNDATION WORKS ON ADJACENT STRUCTURES AND LAND

7.2.1 ASSESSMENT OF THE EFFECT OF FOUNDATION WORKS

The effect of the foundation works on surrounding land, structures and services should be assessed. The assessment should include :

(a) a detailed report on the structural conditions of all surrounding buildings, structures and existing services and the geotechnical condition of the surrounding land likely to be affected by the foundation works;

(b) estimation of the effect on and the response of these buildings, structures, services and land as a result of foundation works; and

(c) proposals of the preventive measures, monitoring scheme and contingency plans to be undertaken before and during construction.

7.2.2 SHORING AND UNDERPINNING

Details of any shoring or underpinning to any building, structure, land or services required as precautionary measures should be included in the foundation proposal.

7.2.3 MONITORING PLAN

Where the construction of a foundation may affect any building, structure, land, street or services, a monitoring plan should be provided. It should contain :

(a) sufficient monitoring stations for the detailed monitoring of movement and vibration in any building, structure, land, street or services;

(b) sufficient piezometers for the detailed monitoring of the ground water conditions;

(c) the frequency at which the readings will be recorded or taken;

(d) the action levels and the contingency measures to be undertaken.

The construction activities on site should also be properly recorded so that they may be correlated with the monitoring readings if necessary. Such correlation can often explain why some apparently abnormal readings are recorded.

The action level for a particular type of services should be agreed with the relevant government department or utility company and should be based on the amount of movement that services could tolerate.

7.2.4 DEWATERING

Dewatering may cause undue settlement of buildings, structures, streets, land and underground services if not properly designed and carried out. Where dewatering is to be undertaken, an assessment of the effects of dewatering on the adjoining buildings, structures, streets, land and underground services should be made. Recharging should be considered where appropriate. A monitoring scheme in accordance with the provisions of Clause 7.2.3 should be provided.

The groundwater table during construction should be properly controlled such that it would be maintained within the limits permitted in the dewatering design. In case the groundwater table is lowered below the permitted limits, the designer should be informed and appropriate action such as suspension of the dewatering should be taken immediately.

7.2.5 VIBRATION

Vibration caused by foundation works should not induce cracks or other damage to any building, structure, land, street or services.

Where protection of historic buildings or structures is required, stringent requirements on vibration control are usually imposed by the appropriate authorities. Where protection of railway is required, reference should be made to the requirements stipulated in the relevant PNAP.

For protection of buildings in general, the vibration should not cause a peak particle velocity of ground movement exceeding 15 mm/sec.

Where necessary, e.g. the presence of any hospital with sensitive equipment, old masonry building, dilapidated structure or delicate utility sensitive to vibration, a test pile should be carried out to establish the tolerable vibration induced by the proposed method of driving. Measures to reduce vibration such as pre-boring for pile installation, controlling the number of piles being driven at any one time, etc. should be implemented where necessary.

7.2.6 BLASTING

If blasting is to be carried out as part of the foundation works, it must be properly designed and controlled such that it will not adversely affect the stability of any adjacent slope, retaining wall, building, structures and services through ground vibrations or other effects. Adequate measures must also be provided to protect the safety of the workers and the public against possible flyrock.

When blasting is to be adopted for foundation works, reference should be made to the following :

(a) Building (Construction) Regulation 23;

(b) Dangerous Goods (General) Regulation 46;

(c) PNAP 77 - Mass Transit Railway Protection (d) PNAP 279 – Kowloon-Canton Railway Protection (e) PNAP 178 - Control of Blasting; and

(f) General Specification for Civil Engineering Works.

7.3 FOUNDATION RECORDS AND REPORTS

The Building Authority requires the submission of record plans, report and a specified form certifying the completion of the foundation works by the relevant personnel upon completion of the works.

Foundation Record Plan

Record plans should include details of the characteristic features of the site and the identification, location, size, depth and level of each foundation unit as constructed.

Foundation Report

The report should include :

(a) the date of construction/installation, the quality and quantity of materials used and any necessary test on the bearing strata for each foundation unit;

(b) the driving performance for driven piles; and

(c) the concrete test cube results, excavation records, predrilling and post-installation proof drilling records for bored piles.

7.4 PILE CONSTRUCTION

7.4.1 DRIVING TEST AND TRIAL PILE

For driven piles, test driving should be conducted to verify the design assumptions made for the piles and founding strata prior to driving any other piles.

Where special ground conditions exist or when a new type of piles is to be used, trial piles may be required to be carried out before the installation of other working piles in order to verify the design assumptions and the performance of the pile. The trial piles should also be tested by the imposition of test loads in accordance with the procedures and criteria specified in Clause 8.4 upon installation.

7.4.2 PRE-DRILLING

For piles founded on rock or rock socket, sufficient pre-drilling should be carried out to identify the depth and quality of the founding rock. Such pre-drilling should be sunk at least 5 m into the category of rock specified for founding or forming of rock socket, or the designed rock socket length of the pile, whichever is the deeper.

For large diameter bored piles, barrettes and the like, pre-drilling should be carried out for each pile.

For mini-piles, socketed steel H-piles, steel H-piles driven to bedrock and similar small diameter bored piles founding on rock or rock socket, pre-drilling should be carried out such that the tip of every pile should be within 5 m distance from a pre-drilling hole. The pre-pre-drilling should be sunk at least 5 m into the category of rock specified for founding or forming of rock socket, or the designed rock socket length of the nearest pile, whichever is the deeper.

7.4.3 POST CONSTRUCTION PROOF DRILLING

Large Diameter Bored Piles, Barrettes and the Like

To ascertain the soundness of the interface, core-drilling should be carried out at the concrete/rock interface for each of the large diameter bored piles, barrettes and the like. The core-drilling should cover at least 1 m above and below the interface.

Socketed H-piles, H-piles driven to bedrock and Mini-piles

To verify the quality of rock for founding or forming of rock socket, additional proof drill holes should be sunk at least 5 m below the as-built top level of the rock socket, or the as-built rock socket length of the nearest pile, whichever is the deeper. The recommended number of such drill holes should be at least 2 for sites with 100 or less piles; or 1% of the number of piles for sites with more than 100 piles (any fraction of a drill hole so calculated should be construed as one additional drill hole). An assessment report with a rockhead contour plan based on the ground investigation, the pre-drilling and the post-installation drilling is required when submitting the piling record plan to the Building Authority.

7.4.4 PROOF TESTS

In accordance with the Building (Construction) Regulation proof tests are required to be conducted on foundation works. The procedures and criteria for proof tests described in Chapter 8 should generally be followed.

Alternative procedures and acceptance criteria other than test loading or core-drilling with sound justification based on recognized foundation engineering principles and relevant to a particular site and building may also be adopted, provided that the following are submitted to the Building Authority to demonstrate the suitability of the proposed method of testing :

(a) relevant recognized engineering principles for the proposed proof test;

(b) detailed procedures of testing and acceptance criteria;

(c) interpretation of the test results; and

(d) any verification tests performed to justify the parameters to be used in the proof test.

7.4.5 FURTHER ON SITE TESTS

Whenever doubt exists as to the design assumption or load carrying capacity of any pile foundation, further on site tests should be carried out.

7.5 GROUND TREATMENT

Where improvement of the load carrying capacity of the ground is to be achieved by ground treatment, adequate proof of the suitability of the method and materials to be used should be given. Where ground treatment has been carried out, adequate tests of the treated ground should be carried out.

Where the ground treatment may affect any building, structure, land, street or services, adequate precautionary measures shall be taken.

7.6 CONTROL OF NUISANCE

Construction site of foundation works may cause environmental nuisance affecting not only the workers on site but also occupants of adjoining buildings and the general public. Appropriate steps should be taken to prevent such nuisance.

(1) Noise

Reference should be made to the Noise Control Ordinance (CAP 400) for detailed requirements in this respect.

In particular, a permit from the Environmental Protection Department is required for carrying out percussive piling operation.

Adequate measures to reduce noise should be taken to meet the requirements of the Environmental Protection Department, if necessary.

(2) Smoke and fume

Reference should be made to the Air Pollution Control Ordinance (CAP 311) for detailed requirements in this respect.

Emission of excessive black smoke or fume from diesel hammer is actionable under the Air Pollution Control Ordinance (CAP 311) which is enforced by Environmental Protection Department.

(3) Waste water and chemical waste

Reference should be made to the Public Health and Municipal Services Ordinance (CAP 132) for detailed requirements in this respect.

In particular, muddy water or chemical waste from a construction site should not be discharged into the drainage system. Any contractor acting against this will be required by the Drainage Services Department to indemnify the Government against costs due to such act.

(4) Vibration

In addition to the provision of Clause 7.2.5, vibration should not create unacceptable discomfort to the occupants of nearby buildings.

7.7 FOUNDATION WORKS IN SCHEDULED AREAS

For foundation works in the Scheduled Areas, there are usually special requirements imposed by the Building Authority. These may include settlement monitoring, control on vibration and performance review of the foundation works. Before commencement of the works, reference should be made to the relevant PNAP issued by the Building Authority.